Mat-immobilized-electrolyte type, Pb-acid batteries include at least one galvanic cell element comprising opposite polarity electrodes (e.g., positive and negative plates) separated one from the other by a porous, absorbent mat (e.g., fibrous polypropylene, Dynel, glass, etc.). The mat is typically compressed between the electrodes and capillarily immobilizes the battery's electrolyte within the cell element. One such battery is described in U.S. Pat. No. 3,853,626 filed Dec. 20, 1968 in the names of Corbin et al and assigned to the assignee of the present invention.
Mat-immobilized-electrolyte batteries may be either of the "flooded" electrolyte type or "starved" electrolyte type. Flooded-electrolyte-type batteries have the mat and electrodes saturated with electrolyte in that the volume of the electrolyte substantially equals or exceeds the void volume of the pores within the mat and the electrodes. Starved-electrolyte-type batteries, on the other hand, have considerably less electrolyte volume than the void volume of the pores within the mat and the electrodes. So-called gas-recombinant batteries, for example, operate in a starved electrolyte condition wherein electrolyte volume is only about 80% to 85% of the available void volume within the mat and electrodes and thereby provide sufficient void space therein to permit migration of oxygen from the positive electrode directly to the negative electrode for consumption thereat.
Conventional (i.e., mat-free) lead-acid storage batteries are commonly manufactured by placing the cell elements in the battery container in the unformed (i.e., uncharged condition) state wherein the positive and negative electrodes both comprise essentially lead oxide. Thereafter H.sub.2 SO.sub.4 forming acid is metered into the cell compartment atop the elements until the compartment is filled. The electrodes are subsequently electrolytically formed (i.e., initially charged) by passing a forming current therethrough. In some cases, the residual forming acid is then dumped and fresh acid substituted therefor as the working electrolyte. In other instances, the forming acid has sufficient residual strength to remain in the battery as the working electrolyte. Between the time the forming acid is added and the formation process begins, the sulfuric acid reacts exothermically with the lead oxide to form lead sulfate in both the positive and negative electrodes in a process often referred to as "pickling". The initial pickling reaction is quite vigorous and results in the generation of considerable heat and gas. The initial pickling reaction subsides considerably after about the first minute or so following acid addition as the acid weakens (i.e., becomes more dilute), the electrodes become sulfated and the lead oxide available for reaction decreases.
Electrolytic formation of the battery follows pickling and involves passing forming current through the battery to convert the lead sulfate in the electrodes to lead dioxide in the positive electrodes, lead in the negative electrodes and to reconstitute sulfuric acid in the electrolyte. The electrolytic formation process adds considerable heat to the battery thereby causing the temperature of the elements to increase significantly. High element temperatures during formation causes the gassing overvoltage of the electrodes to drop which in turn undesirably causes excessive gassing. Hence it is desirable to have the element temperature as low as possible at the beginning of formation so that undesirably high temperatures are not reached during formation. In mat-free batteries, the batteries can cool sufficiently by standing for about 30 to 60 minutes following pickling so as not to significantly affect gassing. Batteries containing glass mats however retain the heat for significantly longer periods of time.
In conventional, mat-free batteries excess gassing during formation, while undesirable, can nonetheless be tolerated as the gas can readily escape the element from between the plates. For mat-containing batteries, however, gassing during formation must be kept to a minimum in order to prevent any gas from becoming entrapped within the mat, causing so-called "dry spots" therein where little or no acid is present. In this regard, dry spots, whether formed during the pickling or formation reactions, will not only interfere with the formation reaction by causing incomplete formation in some regions of the electrodes and increased current densities in other regions thereof, but ultimately results in undesirable lead treeing (i.e., dendrite growth) between the plates, reduced battery capacity and cold cranking performance, as well as shortened cycle life.
Filling mat-immobilized-electrolyte type batteries with electrolyte by dispensing the electrolyte atop the cell element after the cell elements have been placed in their container can have a detrimental affect on the performance of the battery. In this regard, when so filled, the mats often: make it difficult to obtain uniform distribution of acid throughout the cell element; result in boiling of the electrolyte within the cell element during pickling; and/or result in mats containing pockets of trapped gases (i.e., dry spots). More specifically, without precautions (e.g., special container designs), electrolyte introduced into the container atop the cell element percolates down through the cell element from the top thereof, thereby trapping air within the element as well as preventing the ready escape of the gases and heat generated during the pickling reaction. As a result, not only are dry spots prevalent, but the temperature of the element is elevated to an undesirably high level and retains the heat for a prolonged period of time. Moreover, the concentration of the electrolyte within the cell element tends to vary from one location to the next. One reason for this is the stratification that occurs by virtue of the electrolyte front's descending down through the cell element. As the electrolyte front (i.e., the initial few centimeters of the electrolyte wave moving into the element) advances into the element, it is more rapidly depleted of its H.sub.2 SO.sub.4 content then is the electrolyte tracking behind the front. As a result, by the time the liquid front reaches the bottom of the cell element, it has much lower sulfuric acid concentration than the acid near the top of the element. This concentration imbalance affects the conductivity of the electrolyte at different locations in the element, which in turn affects current density distribution, dendrite formation and corrosion rate of the electrode's supporting grid structure in those regions of the element where the electrolyte is more dilute. Another reason for electrolyte concentration variations is the formation of dry spots (i.e., trapped gases) discussed above. Eventually, acid will infiltrate even into the dry spots, at least to some extent, but is quickly consumed by the unformed PbO in the regions of the electrodes adjacent the dry spots, resulting in pockets of low concentration acid within the element. Finally, because mat-immobilized electrolyte elements do not take up electrolyte as quickly as mat-free elements, care must be taken to meter the electrolyte into the container atop the element at a sufficiently slow rate as to preclude overflow thereof from the top of the container above the cell element.